Coating Liners??

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NorthwoodsRockets
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Coating Liners??

Postby NorthwoodsRockets » Tue Dec 29, 2015 9:39 pm

So, I want to cast my first 76 mm grains and I have heard people talking about coating their liners with R45.
I have another week of vacation from school and am hoping to cast four 76 mm grains in order to use three in an L. I have heard people talking about sanding and coating large motor casting tubes with R45 and curative to avoid separation of the grain from the casting tube.
Is this standard operating procedure? Or, when is it necessary?
FYI, this motor is going in my L3 Coke Bottle,
DSCN1254.JPG
so acceleration isn't going to be remarkable. (Maybe remarkably low.)
Thanks for the guidance,
Matt

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patelldp
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Re: Coating Liners??

Postby patelldp » Tue Dec 29, 2015 11:31 pm

I think it will do what you want...if you want to avoid burning on the OD of the grains. It's probably not necessary if you've never had issues with debonding at the propellant/casting tube interface.

I know people that swear by it, but I'm not convinced it's necessary.
Dan Patell
TRA 10904 L3

mdutch
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Re: Coating Liners??

Postby mdutch » Wed Dec 30, 2015 8:12 am

When I first started doing my own motors one of the people in the mix party I was at was making some wimpy red and said he did it because the formula was dry enough to warrant doing that.(Later I was able to remedy this issue by adding .5% Lecithin to the mix) Then later I was told by Pat G. that most mixes have enough liquid in them to bond with the liners and that it is when you start making larger motors with 2" or more between the core and the casting tube that you should coat the CT. (IE like when you start making 155mm grains) I have never had an issue with bonding on any of my 75mm of 98mm motors by the way. I would recommend that if your mix comes out sandy or seems to be a bit dry to then coat the casting tubes. Then the next time you make the formula to add .5% Lecithin to it. ( Lecithin is a viscosity reducer and should loosen up the mix without effecting the performance.)
Mike Dutch
URRG & MRSI Founder
TAP L3CC
"I just want to fly ROCKETS"

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patelldp
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Re: Coating Liners??

Postby patelldp » Wed Dec 30, 2015 8:28 am

mdutch wrote:When I first started doing my own motors one of the people in the mix party I was at was making some wimpy red and said he did it because the formula was dry enough to warrant doing that.(Later I was able to remedy this issue by adding .5% Lecithin to the mix) Then later I was told by Pat G. that most mixes have enough liquid in them to bond with the liners and that it is when you start making larger motors with 2" or more between the core and the liner that you should coat the liner. (IE like when you start making 155mm grains) I have never had an issue with bonding on any of my 75mm of 98mm motors since then. I would recommend that if your mix comes out sandy or seems to be a bit dry to then coat the casting tubes. Then the next time you make the formula to add .5% Lecithin to it. ( Lecithin is a viscosity reducer and should loosen up the mix without effecting the performance.)


Lecithin is essentially an oily substance to aid in the mixing of propellant, yes. It's used frequently in the food industry. That said, you're probably better off eliminating the lecithin and increasing your DOA content or adding finer particles. Lecithin won't help with "sandy" or "dry" propellant, that's a much bigger issue and requires some content changes (elimination of 400um AP?) and additional mixing to properly coat your particles.

I think that this fear of outer-diameter burning in rocket motors is perpetuated by the appearance of grains recovered from a CATO. Often times the grains appear to have external burning down the spirals on the OD. While it's certainly possible that that could have occurred, it's also nearly impossible to learn anything from CATO'd grains. This is because a lot of unknowns occur, specifically increased pressure inside of the chamber and ultimately continued combustion outside of the combustion chamber.

My advice is this...make sure that the ID of your casting tube is clean. Thoroughly mix your propellant to properly coat your particles. Make the DOA 20-25% of your binder percentage. Introduce some smaller sized AP to your propellant. Ditch the Lecithin (I have part of a bottle, anyone want it?).
Dan Patell
TRA 10904 L3

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patelldp
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Re: Coating Liners??

Postby patelldp » Thu Feb 04, 2016 1:57 pm

How'd these grains turn out?
Dan Patell
TRA 10904 L3

PGagnon
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Re: Coating Liners??

Postby PGagnon » Thu Feb 04, 2016 6:28 pm

patelldp wrote:
mdutch wrote:When I first started doing my own motors one of the people in the mix party I was at was making some wimpy red and said he did it because the formula was dry enough to warrant doing that.(Later I was able to remedy this issue by adding .5% Lecithin to the mix) Then later I was told by Pat G. that most mixes have enough liquid in them to bond with the liners and that it is when you start making larger motors with 2" or more between the core and the liner that you should coat the liner. (IE like when you start making 155mm grains) I have never had an issue with bonding on any of my 75mm of 98mm motors since then. I would recommend that if your mix comes out sandy or seems to be a bit dry to then coat the casting tubes. Then the next time you make the formula to add .5% Lecithin to it. ( Lecithin is a viscosity reducer and should loosen up the mix without effecting the performance.)


Lecithin is essentially an oily substance to aid in the mixing of propellant, yes. It's used frequently in the food industry. That said, you're probably better off eliminating the lecithin and increasing your DOA content or adding finer particles. Lecithin won't help with "sandy" or "dry" propellant, that's a much bigger issue and requires some content changes (elimination of 400um AP?) and additional mixing to properly coat your particles.

I think that this fear of outer-diameter burning in rocket motors is perpetuated by the appearance of grains recovered from a CATO. Often times the grains appear to have external burning down the spirals on the OD. While it's certainly possible that that could have occurred, it's also nearly impossible to learn anything from CATO'd grains. This is because a lot of unknowns occur, specifically increased pressure inside of the chamber and ultimately continued combustion outside of the combustion chamber.

My advice is this...make sure that the ID of your casting tube is clean. Thoroughly mix your propellant to properly coat your particles. Make the DOA 20-25% of your binder percentage. Introduce some smaller sized AP to your propellant. Ditch the Lecithin (I have part of a bottle, anyone want it?).




Dan

The smaller the particle size the more surface aria that a binder has to cover assuming the weight remains the same for the particle.
Example, If you cut a ball in half you now have 2 extra surfaces to coat.

I don't think that reducing the particle size would be a good move. unless you want a faster burn rate. ;)

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tfish
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Re: Coating Liners??

Postby tfish » Thu Feb 04, 2016 7:02 pm

I agree.

I took out 3% AP out a mellow formula and added 3% 'powder' from a dry chem fire extinguisher (trying to slow the formula down..cheaply) and the motor let go at ignition. I had not noticed how dry the formula had become. I was also playing around with some sparky motors and had your basic high binder content (22-25%?) and tried using some Ti late turnings in place of Ti sponge I had been using. That motor let go to. Seems there's a lot of surface area to weight ratio on Ti lathe turnings. Even at 75% solids a formula can fall into a dry mix category.

The formula that I do pre-treat the casting tubes on is Wimpy Red. It seems like it's needed there, especially if pulling a lot of G's.

Tony

PGagnon
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Re: Coating Liners??

Postby PGagnon » Thu Feb 04, 2016 9:41 pm

The only time you really need to coat the casting tube is when your using a DRY mix.
I made and launched a N-4256 at balls. Pulled 36 Gs, Mach 2.4 in just shy of a mile. ( 98mm).
I did NOT pretreat the casting tube. With the larger particle size, At 82% solids, I had enough wet to do the job.

JD13x
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Re: Coating Liners??

Postby JD13x » Thu Feb 04, 2016 10:43 pm

I have never pre-coasted the casting tubes with R45 before.
I would believe that if you were to make an 85% solids batch or higher it would warrant it.
Another reason that I could see someone doing this would be: IF you are using some exotic or non-porous casting tubes.
People may also do this if: they are experiencing burn through's or burning along the spirals of the casting tubes.

Most of the time these are long L/D motors.

JD


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